This invention is directed to adhesive and sealant compositions which contain hydrogenated diblock copolymers. More particularly, the invention is related to such compositions containing triblock or radial or star hydrogenated block copolymers as well as the hydrogenated diblocks.
Block copolymers have been employed in adhesive compositions for many years, primarily because of their high cohesive strengths and their ability to crosslink without a chemical vulcanization step. Block copolymers such as those described in U.S. Pat. No. 3,239,478 are either linear or radial or star styrene-butadiene or styrene-isoprene block copolymers. The high cohesive strength of these polymers is often a detrimental quality in certain applications. In the past, cohesive strength was reduced by adding an unhydrogenated styrene-isoprene diblock copolymer to the primary block copolymer to lower the cohesive strength and give less elasticity and better conformability. The diblocks had to be unhydrogenated in order to provide sufficient tackiness.
These conventional block copolymers when used in adhesives tend to degrade in processing and/or over time because they are unsaturated in the main rubber chain. These unsaturation sites are reactive sites which are vulnerable to attack, such as by free radicals created by oxidation, ultraviolet light or mechanical action. As a result, the polymer chain may be severed by chain scission, reducing the molecular weight and those properties which are sensitive to molecular weight. Alternatively, the unsaturation sites may be subjected to grafting and crosslinking reactions which raise the molecular weight and undesirably stiffen the polymer making it unprocessable or ineffective as an adhesive. Hydrogenating the conventional unsaturated base polymers creates a nonpolar polymer which, although more stable, is difficult to tackify with resin additives and which is therefore inferior to unhydrogenated polymers in some applications, including pressure sensitive adhesives. It has always been the conventional wisdom that hydrogenated diblocks would suffer from the same problems and would not be acceptable in such adhesives.
The present invention offers a solution to some of these problems while still providing adequate adhesive properties, including lower viscosity than the base polymer alone, higher resistance to creep, better tack, and/or improved compatibility with certain substrates. It does so by providing an additional component which assists in tackifying the hydrogenated base polymer.